func (w testNonEmptyWriter) Write(b []byte) (int, error) {
	if len(b) == 0 {
		return 0, errors.New("unexpected empty Write call")
	}
	return w.Writer.Write(b)
}

func TestFileWriter(t *testing.T) {
	type (
		testWrite struct { // Write(str) == (wantCnt, wantErr)
			str     string
			wantCnt int
			wantErr error
		}